High efficiency photovoltaic device employing cadmium sulfide telluride and method of manufacture

ABSTRACT

A photovoltaic device is disclosed including at least one Cadmium Sulfide Telluride (CdS x Te 1-x ) layer as are methods of forming such a photovoltaic device.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application No.61/792,153, filed on Mar. 15, 2013 and U.S. Provisional Application No.61/792,233, filed on Mar. 15, 2013, which are hereby fully incorporatedby reference.

TECHNICAL FIELD

This disclosure relates generally to photovoltaic (PV) devices, and morespecifically, to high efficiency thin film PV devices and methods formanufacturing such devices.

BACKGROUND

Photovoltaic (PV) devices are PV cells or PV modules containing aplurality of PV cells or any device that converts photo-radiation orlight into electricity. Generally, a thin film PV device includes twoconductive electrodes sandwiching a series of semiconductor layers. Thesemiconductor layers provide an n-type window layer in close proximityto a p-type absorber layer to form a p-n junction. During operation,light passes through the window layer, and is absorbed by the absorberlayer. The absorber layer produces photo-generated electron-hole pairs,the movement of which, promoted by a built-in electric field generatedat the p-n junction, produces electric current that can be outputthrough the two electrodes.

There is an ever present desire to increase the efficiency of a PVdevice in the photoconversion process.

DESCRIPTION OF DRAWINGS

FIG. 1 shows a portion of a PV device in accordance with a disclosedembodiment.

FIG. 2 shows a portion of a PV device in accordance with a disclosedembodiment.

FIG. 3 shows a portion of a PV device in accordance with a disclosedembodiment.

FIG. 4 shows a portion of a PV device in accordance with a disclosedembodiment.

FIG. 5 shows a portion of a PV device in accordance with a disclosedembodiment.

FIG. 6 shows a portion of a PV device in accordance with a disclosedembodiment.

FIG. 7 shows a portion of a PV device in accordance with a disclosedembodiment.

FIG. 7A shows a portion of a PV device in accordance with a disclosedembodiment.

FIG. 7B shows a portion of a PV device in accordance with a disclosedembodiment.

FIG. 7C shows a portion of a PV device in accordance with a disclosedembodiment.

FIG. 7D shows a portion of a PV device in accordance with a disclosedembodiment.

FIG. 7E shows a portion of a PV device in accordance with a disclosedembodiment.

FIG. 8 shows a portion of a PV device in accordance with a disclosedembodiment.

FIG. 9 is a schematic diagram of a VTD coater which can be configured toproduce PV devices, in accordance with disclosed embodiments.

DETAILED DESCRIPTION

Since light has to pass through the window layer to be converted toelectricity, it is desirable to have a thin window layer which allowslight to pass therethrough to the absorber layer. The more light thatpasses through the window layer, the more efficient the device. Thus,one method the included embodiments use to increase devicephoto-conversion efficiency is to use a window layer that is thin,dis-continuous, or even absent.

Another method the included embodiments use to enhance devicephoto-conversion efficiency is to reduce defect areas. Specifically, onefactor that may limit thin-film photo-conversion efficiency is thenumber of photo-generated electron-hole pairs (i.e., carriers) that aretrapped and then recombined before they are output as electricity by thedevice. In some instances, carriers may get trapped at structuraldefects such as defective grain or surface boundaries within or betweenvarious layers of the device.

For example, the semiconductor absorber layer is formed of grains, alsoknown as crystallites. Crystallites are small, microscopic crystals,where the orientation of the crystal lattice within the crystallite isthe same. But, a defect exists where the orientation of the crystallattice changes from one grain to another. Hence, the crystallites thatmake up the absorber layer may be said to have defective grainboundaries where crystallites on each side of the boundary areidentical, except in crystal orientation. Similarly, defects, such assurface defects, may also occur at interfaces between materials due tolattice mismatch. That is, each material has a different distancebetween unit cells in its crystal lattice.

In any case, the larger the grains that make up the absorber layer, thelesser the number of grain boundary defects present in the absorberlayer. Similarly, the smaller the lattice mismatch between interfacematerials of the enclosed PV device embodiments, the lesser the numberof surface defects. The fewer the defects in the enclosed PV deviceembodiments, the higher the conversion efficiency due to fewerelectron-hole recombinations which can occur at defect locations.

One method of decreasing the number of defects in the enclosedembodiments is to subject the absorber layer to a cadmium chloride(CdCl₂) activation treatment. Alternative compounds for the activationtreatment can also be used such as, for example, NHCl₂, ZnCl₂, TeCl₂, orother halide salts. The CdCl₂ activation treatment increases the size ofthe grains or crystals that make up the absorber layer and thus reducesthe number of grain boundaries available to trap carriers. Deviceefficiency may thus be enhanced.

For example, a thin-film PV device may have a window layer formed ofcadmium sulfide (CdS) and an absorber layer formed of cadmium telluride(CdTe). The CdCl₂ activation treatment includes applying CdCl₂, forexample, in liquid or vapor form, to the CdTe of the absorber layer, andthen annealing the absorber layer at a particular anneal temperature,for example between about 400° C. to about 450° C., for a particularanneal time, for example, from about 5 minutes to about one hour. Theanneal temperature is generally high enough and the anneal time longenough to promote recrystallization of the CdTe crystallites.

The recrystallization of the CdTe material can take two forms or acombination of the following two forms: (1) intragrain or primaryrecrystallization (recrystallization that changes crystallite grainorientation); and (2) intergrain, or secondary recrystallization(recrystallization resulting from grain coalescence).

The primary crystallization leads to adjacent grains, which wereoriented differently, to now be oriented in the same direction. Hence,primary crystallization results in a lesser number of defectiveboundaries being available to trap carriers therein. By contrast, thesecondary recrystallization results in grain growth as smaller grainscoalesce into larger ones. Thus, it too, leads to a reduced number ofgrain boundaries, which could contain defects.

Further, in addition to reducing the number of defective grainboundaries in the absorber layer, the CdCl₂ activation treatment alsorepairs some of the defects in the grain and surface boundaries. This isdone through the incorporation of chlorine atoms (or ions) from theCdCl₂ into the CdTe absorber layer. Other mechanisms believed to repairor passivate such defects include the formation of doping complexeswithin the absorber layer created by cadmium vacancies, theincorporation of chlorine atoms to occupy tellurium sites, andinter-diffusion of materials between the absorber layer and thesemiconductor window layer.

The CdCl₂ and the heat from the CdCl₂ annealing, while being beneficialin reducing the number of defective grain boundaries in the absorberlayer may also promote chemical fluxing. Fluxing occurs when a chemicalelement from one layer of a photovoltaic device, where it is in highconcentration, flows into another layer where there is a lowconcentration, or where it is not.

In this case, the CdCl₂ activation treatment may increase the mobilityof sulfur atoms from a CdS window layer, overly thinning the layer, andin extreme situations, entirely removing it in some areas where the p-njunction is degraded or lost. The degradation of the absorber-window p-njunction is generally thought to be undesirable. Some PV devices usethicker window layers to prevent loss of p-n junctions during CdCl₂activation treatment, however this too is undesirable because thethicker window layer prevents more light from reaching the absorberlayer.

Thus, various disclosed embodiments incorporate a depositedCdS_(x)Te_(1-x) material (where x is greater than zero and less thanone), which is an alloy of cadmium (Cd), sulfur (S), and tellurium (Te),to allow PV device configurations which: allow more incident photons toreach the absorber layer; control fluxing of the window layer to allowfor more aggressive halide salt, i.e. CdCl₂, activation treatment;passivate other defects including surface defects; and/or are lesscomplicated or require less materials to produce.

For illustrative purposes, embodiments are described below withreference to a thin film PV device. However, it should be understoodthat the embodiments may apply to PV devices other than thin film PVdevices.

A photovoltaic device according to one example of the invention caninclude an absorber layer, a non-continuous window layer, and aCdS_(x)Te_(1-x) layer between the non-continuous window layer and theabsorber layer, where x is greater than 0 and less than 1. In anotheraspect of the invention, the photovoltaic device can be configuredwherein x in the CdSxTe_(1-x) is between about 0.01 and about 0.3, orfor example, wherein x is between about 0.01 and about 0.15, or forexample wherein x is between about 0.02 and about 0.10. In one exampleof the invention the photovoltaic device non-continuous window layer cancomprise discontinuous segments of CdS material. In another example ofthe invention, the photovoltaic device absorber layer can comprise CdTe.In yet another example of the invention, the non-continuous window layercan be configured to have an average segment thickness of greater than 0nm and up to about 50 nm.

In another example, a CdS_(x)Te_(1-x) layer can have a thickness ofabout 30 nm to about 1000 nm, or have a thickness of about 400 nm toabout 600 nm. In another example, the photovoltaic device can beconfigured to have and absorber layer with a thickness from about 1000nm to about 8000 nm. In one aspect of the invention, a non-continuouswindow layer comprises CdS segments which have an average thickness ofless than about 50 nm, a CdS_(x)Te_(1-x) layer has a thickness in therange of about 30 nm to about 1000 nm, and an absorber layer comprisesCdTe and has a thickness in the range of about 1000 nm to about 8000 nm.In another aspect of the invention the photovoltaic device can furthercomprise a first electrode and a buffer layer between the firstelectrode and the non-continuous window layer. In one example, thebuffer layer comprises an n-type semiconductor material. In anotherexample, the buffer layer can comprise at least one of a materialselected from the group consisting of: SnO₂ and ZnO.

In another aspect of the invention, a CdS_(x)Te_(1-x) layer can have asulfur concentration gradient over its thickness such that aconcentration of sulfur is greater at portions closest to anon-continuous window layer than at portions closest to the absorberlayer. In another aspect of the invention, a CdS_(x)Te_(1-x) layercomprises a first sub-layer adjacent anon-continuous window layer and asecond sub-layer adjacent the first sub-layer. In one example, thephotovoltaic device of claim 16 wherein the first sub-layer has a highersulfur concentration than the second sub-layer. In one aspect of theinvention, at least one of a first and a second sub-layers has aconsistent sulfur content over its thickness. In another aspect of theinvention at least one of a first sub-layer and a second sub-layer has agraded sulfur content over its thickness. In another example, each ofthe first and second sub-layers has a thickness of greater than 0 nm andless than or equal to about 500 nm.

A photovoltaic device, according to an example of the invention caninclude a semiconductor absorber layer, a semiconductor window layer ona first side of the semiconductor absorber layer, and a CdS_(x)Te_(1-x)layer on a second side of the semiconductor absorber layer, wherein x isgreater than 0 and less than 1. In another aspect of the invention, thephotovoltaic device can be configured wherein x in the CdSxTe_(1-x) isbetween about 0.01 and about 0.3, or for example, wherein x is betweenabout 0.01 and about 0.15, or for example wherein x is between about0.02 and about 0.10. In another aspect of the invention aCdS_(x)Te_(1-x) layer can be configured to be about 50 nm to about 1000nm thick. In another aspect of the invention the semiconductor windowlayer is about 0 nm to about 100 nm thick. In a further aspect of theinvention, the semiconductor absorber layer comprises sulfur provided bythe CdS_(x)Te_(1-x) layer. In one example, the CdS_(x)Te_(1-x) layer isa copper-doped CdS_(x)Te_(1-x) layer. In one aspect of the invention acopper-doped CdS_(x)Te_(1-x) layer comprises about 1 ppm copper to about50 ppm copper.

A photovoltaic device according to an example of the invention caninclude a semiconductor window layer, a semiconductor absorber layeradjacent to the semiconductor window layer, and a CdS_(x)Te_(1-x) layerwithin the semiconductor absorber layer, wherein x is greater than 0 andless than 1. In one aspect of the invention, the photovoltaic device canbe configured wherein x in the CdSxTe_(1-x) is between about 0.01 andabout 0.3, or for example, wherein x is between about 0.01 and about0.15, or for example wherein x is between about 0.02 and about 0.10. Inone aspect of the invention the CdS_(x)Te_(1-x) layer can be about 50 nmto about 1000 nm thick. In another aspect of the invention thesemiconductor absorber layer comprises sulfur provided by theCdS_(x)Te_(1-x) layer. In yet another aspect of the invention theCdS_(x)Te_(1-x) layer is a copper-doped CdS_(x)Te_(1-x) layer. In oneexample, the copper-doped CdS_(x)Te_(1-x) layer comprises about 1 ppmcopper to about 50 ppm copper.

A photovoltaic device according to another example of the inventionincludes a semiconductor absorber layer, a semiconductor window layer ona first side of the semiconductor absorber layer, a firstCdS_(x)Te_(1-x) layer on the first side of the semiconductor absorberlayer and between the semiconductor window layer and the semiconductorabsorber layer, and a second CdS_(x)Te_(1-x) layer on a second side ofthe semiconductor absorber layer, wherein x in each of the first andsecond CdS_(x)Te_(1-x) is independently greater than 0 and less than 1.In one aspect of the invention x is can be between about 0.01 to about0.30 in at least one of the first CdS_(x)Te_(1-x), layer and the secondCdS_(x)Te_(1-x) layer. In another aspect of the invention wherein x isbetween about 0.01 to about 0.15 in at least one of the firstCdS_(x)Te_(1-x) layer and the second CdS_(x)Te_(1-x) layer. In yetanother aspect of the invention, x is between about 0.02 to about 0.10in at least one of the first CdS_(x)Te_(1-x) layer and the secondCdS_(x)Te_(1-x) layer.

In one configuration, at least one of the first CdS_(x)Te_(1-x) layerand the second CdS_(x)Te_(1-x) layer is about 50 nm to about 1000 nmthick. In one aspect of the invention the semiconductor absorber layercomprises sulfur provided by at least one of the first CdS_(x)Te_(1-x)layer and the second CdS_(x)Te_(1-x) layer. In another aspect of theinvention at least one of the first CdS_(x)Te_(1-x) layer and the secondCdS_(x)Te_(1-x) layer is a copper-doped CdS_(x)Te_(1-x) layer. In oneconfiguration the copper-doped CdS_(x)Te_(1-x) layer comprises about 1ppm copper to about 50 ppm copper.

A photovoltaic device according to another example of the inventionincludes a buffer layer, a semiconductor absorber layer, and aCdS_(x)Te_(1-x) layer between the buffer layer and the semiconductorabsorber layer, wherein x is greater than 0 and less than 1, and whereinthe photovoltaic device does not include a window layer. In one aspectof the invention x is can be between about 0.01 to about 0.30 in atleast one of the first CdS_(x)Te_(1-x) layer and the secondCdS_(x)Te_(1-x) layer. In another aspect of the invention wherein x isbetween about 0.01 to about 0.15 in at least one of the firstCdS_(x)Te_(1-x) layer and the second CdS_(x)Te_(1-x) layer. In yetanother aspect of the invention, x is between about 0.02 to about 0.10.In one configuration, the CdS_(x)Te_(1-x) layer is about 200 nm to about1000 nm thick. In one example, the semiconductor absorber layercomprises sulfur provided by the CdS_(x)Te_(1-x) layer. In one aspect ofthe invention the CdS_(x)Te_(1-x) layer is a copper-dopedCdS_(x)Te_(1-x) layer. In yet another aspect of the invention, thecopper-doped CdS_(x)Te_(1-x) layer comprises about 1 ppm copper to about50 ppm copper. In yet another aspect of the invention the photovoltaicdevice includes an additional sulfur-containing layer.

In one configuration of the photovoltaic device, the additionalsulfur-containing layer is positioned between the CdS_(x)Te_(1-x) layerand the semiconductor absorber layer. In one aspect of the invention,the additional sulfur-containing layer is positioned within thesemiconductor absorber layer. In yet another aspect of the invention theadditional sulfur-containing layer is positioned between thesemiconductor absorber layer and a back contact. In yet another aspectof the invention, x is between about 0.05 and 0.5.

Furthermore, in another example, the additional sulfur-containing layercomprises a material selected from the group consisting of aCdS_(x2)Te_(1-x2) material (where x²<x), CdS, ZnS and ZnO_(x)S_(y).

A photovoltaic device according to another example of the inventionincludes a buffer layer over a front contact and a CdS_(x)Te_(1-x) layerbetween the buffer layer and a back contact, wherein x is greater than 0and less than 1, and wherein the photovoltaic device does not include aseparate semiconductor absorber layer or a separate semiconductor windowlayer. In another aspect of the invention, the CdS_(x)Te_(1-x) layerfunctions as an absorber layer. In yet another aspect of the invention,x can be between about 0.01 to about 0.30. In another example, x isbetween about 0.01 to about 0.15. And in another example, x is betweenabout 0.02 to about 0.10. According to one aspect of the invention x issubstantially constant over an entire thickness of the CdS_(x)Te_(1-x)layer. In another aspect of the invention x changes over a thickness ofthe CdS_(x)Te_(1-x) layer such that x is larger where theCdS_(x)Te_(1-x) layer is nearer the front contact and x is smaller wherethe CdS_(x)Te_(1-x) layer is nearer the back contact. And in anotheraspect of the invention, the CdS_(x)Te_(1-x) layer is graded in sulfurand tellurium content over a thickness of the CdS_(x)Te_(1-x) layer in astep-wise fashion. In yet another aspect of the invention theCdS_(x)Te_(1-x) layer is graded in sulfur and tellurium content over athickness of the CdS_(x)Te_(1-x) layer in a gradual fashion.

In one aspect of the invention the CdS_(x)Te_(1-x) layer can be about 2μm to about 8 μm thick. In another aspect of the invention theCdS_(x)Te_(1-x) layer is a copper-doped CdS_(x)Te_(1-x) layer. In oneconfiguration, the copper-doped CdS_(x)Te_(1-x) layer comprises about 1ppm copper to about 50 ppm copper.

One of the methods of forming a photovoltaic device includes forming afirst contact layer, forming a semiconductor window layer over the firstcontact layer, forming a semiconductor absorber layer over thesemiconductor window layer, forming a CdS_(x)Te_(1-x) layer over thesemiconductor absorber layer, wherein x is greater than 0 and less than1, and forming a second contact layer over the CdS_(x)Te_(1-x) layer. Inone aspect of the method, x is between about 0.01 to about 0.30. Inanother aspect, x is between about 0.01 to about 0.15. And in yetanother aspect of the method, x is between about 0.02 to about 0.10.

In one particular embodiment, the CdS_(x)Te_(1-x) layer is about 50 nmto about 1000 nm thick. In one aspect of the method, the semiconductorabsorber layer comprises sulfur provided by the CdS_(x)Te_(1-x) layer.In one example, the method can include performing an activation stepincluding annealing the semiconductor window layer, semiconductorabsorber layer, and CdS_(x)Te_(1-x) layer in the presence of CdCl₂. Inone example, the CdS_(x)Te_(1-x) layer maintains an interface betweenthe semiconductor window layer and the semiconductor absorber layerduring the activation step. The method can further include doping theCdS_(x)Te_(1-x) layer with about 1 ppm copper to about 50 ppm copper.

A method of forming a photovoltaic device according to another exampleof the invention includes forming a first contact layer, forming asemiconductor window layer over the first contact layer, forming aportion of a semiconductor absorber layer over the semiconductor windowlayer, forming a CdS_(x)Te_(1-x) layer over the portion of thesemiconductor absorber layer, wherein x is greater than 0 and less than1, forming a remainder of the semiconductor absorber layer, and forminga second contact layer over the semiconductor absorber layer. In oneaspect of the method, x is between about 0.01 to about 0.30. In anotheraspect of the invention x is between about 0.01 to about 0.15. In yetanother aspect of the invention x is between about 0.02 to about 0.10.In one particular embodiment the CdS_(x)Te_(1-x) layer is formed toabout 50 nm to about 1000 nm thick. In one section of the invention, thesemiconductor absorber layer comprises sulfur provided by theCdS_(x)Te_(1-x) layer.

The method can further include performing an activation step includingannealing the semiconductor window layer, semiconductor absorber layer,and CdS_(x)Te_(1-x) layer in the presence of CdCl₂. In one aspect of theinvention the CdS_(x)Te_(1-x) layer maintains an interface between thesemiconductor window layer and the semiconductor absorber layer duringthe activation step. The method can also include doping theCdS_(x)Te_(1-x) layer with about 1 ppm copper to about 50 ppm copper.

A method of forming a photovoltaic device according to another exampleof the invention includes forming a first contact layer, &tuning asemiconductor window layer over the first contact layer, forming a firstCdS_(x)Te_(1-x) layer over the semiconductor window layer, forming asemiconductor absorber layer over the first CdS_(x)Te_(1-x) layer,forming a second CdS_(x)Te_(1-x) layer over the semiconductor absorberlayer, wherein x in each of the first and second CdS_(x)Te_(1-x) isindependently greater than 0 and less than 1, and forming a secondcontact layer over the second CdS_(x)Te_(1-x) layer. In one aspect ofthe method x is between about 0.01 to about 0.30 in at least one of thefirst CdS_(x)Te_(1-x) layer and the second CdS_(x)Te_(1-x) layer. Inanother aspect of the method, x is between about 0.01 to about 0.15 inat least one of the first CdS_(x)Te_(1-x) layer and the secondCdS_(x)Te_(1-x) layer. In yet another aspect of the method, x is betweenabout 0.02 to about 0.10 in at least one of the first CdS_(x)Te_(1-x)layer and the second CdS_(x)Te_(1-x) layer.

In one example embodiment, at least one of the first CdS_(x)Te_(1-x)layer and the second CdS_(x)Te_(1-x) layer is about 50 nm to about 1000nm thick. In one aspect of the method the semiconductor absorber layercomprises sulfur provided by at least one of the first CdS_(x)Te_(1-x)layer and the second CdS_(x)Te_(1-x) layer. In another aspect of theinventive method, the method further includes performing an activationstep including annealing the semiconductor window layer, semiconductorabsorber layer, and at least one of the first CdS_(x)Te_(1-x) layer andsecond CdS_(x)Te_(1-x) layer in the presence of CdCl₂. The method can beperformed such that at least one of the first CdS_(x)Te_(1-x) layer andthe second CdS_(x)Te_(1-x) layer maintains an interface between thesemiconductor window layer and the semiconductor absorber layer duringthe activation step. In one aspect of the method, the method includesdoping at least one of the first CdS_(x)Te_(1-x) layer and the secondCdS_(x)Te_(1-x) layer with about 1 ppm copper to about 50 ppm copper.

A method of forming a photovoltaic device according to another exampleof the invention includes forming a first contact layer, forming abuffer layer over the first contact layer, forming a CdS_(x)Te_(1-x)layer over the buffer layer, wherein x is greater than 0 and less than1, and wherein no separate semiconductor window layer is formed, forminga semiconductor absorber layer over the CdS_(x)Te_(1-x) layer, andforming a second contact layer over the semiconductor absorber layer. Inone aspect of the method x is between about 0.01 to about 0.30. Inanother aspect of the method, x is between about 0.01 to about 0.15. Inyet another aspect of the method x is between about 0.02 to about 0.10.In one particular embodiment the CdS_(x)Te_(1-x) layer is about 50 nm toabout 1000 nm thick. In one aspect of the method the semiconductorabsorber layer comprises sulfur provided by the CdS_(x)Te_(1-x) layer.

In one example of the method can include performing an activation stepincluding annealing the semiconductor absorber layer and CdS_(x)Te_(1-x)layer in the presence of CdCl₂. The method can also include doping theCdS_(x)Te_(1-x) layer with about 1 ppm copper to about 50 ppm copper. Inone aspect of the invention, the method can include forming anadditional sulfur-containing layer. In another aspect of the invention,the additional sulfur-containing layer is positioned between theCdS_(x)Te_(1-x) layer and the semiconductor absorber layer. In yetanother aspect of the invention the additional sulfur-containing layeris positioned within the semiconductor absorber layer. In yet anotheraspect of the invention the additional sulfur-containing layer ispositioned between the semiconductor absorber layer and a back contact.In one particular example, x is between about 0.05 and 0.5. In oneaspect of the invention the additional sulfur-containing layer comprisesa material selected from the group consisting of a CdS_(x2)Te_(1-x2)material (where x₂<x), CdS, ZnS and ZnO_(x)S_(y).

A method of forming a photovoltaic device according to another exampleof the invention includes forming a first contact layer, forming abuffer layer over the first contact layer, forming a CdS_(x)Te_(1-x)layer over the buffer layer, wherein x is greater than 0 and less than1, and forming a second contact layer over the CdS_(x)Te_(1-x) layer,wherein no CdS window layer and no CdTe absorber layer are provided. Inone aspect of the invention, the CdS_(x)Te_(1-x) layer functions as anabsorber layer. In one particular example, x is between about 0.01 toabout 0.30. In another example, x is between about 0.01 to about 0.15.And in yet another example, x is between about 0.02 to about 0.10.

In one aspect of the invention x is substantially constant over anentire thickness of the CdS_(x)Te_(1-x) layer. In another aspect of theinvention x changes over a thickness of the CdS_(x)Te_(1-x) layer suchthat x is larger where the CdS_(x)Te_(j), layer is nearer the firstcontact and x is smaller where the CdS_(x)Te_(1-x) layer is nearer thesecond contact. In another aspect of the invention, the CdS_(x)Te_(1-x)layer is graded in sulfur and tellurium content over a thickness of theCdS_(x)Te_(1-x) layer in a step-wise fashion. And in yet another aspectof the invention, the CdS_(x)Te_(1-x) layer is graded in sulfur andtellurium content over a thickness of the CdS_(x)Te_(1-x) layer in agradual fashion. In one particular embodiment, the CdS_(x)Te_(1-x) layeris about 2 μm to about 8 μm thick. The method can include doping theCdS_(x)Te_(1-x) layer with about 1 ppm copper to about 50 ppm copper.

A method of forming a photovoltaic device according to another exampleof the invention includes depositing a CdS_(x)Te_(1-x) layer over a TCOstack deposited on a substrate, wherein x is greater than 0 and lessthan 1, and depositing a semiconductor absorber layer over theCdS_(x)Te_(1-x) layer, wherein the CdS_(x)Te_(1-x) layer and thesemiconductor absorber layer are deposited using a VTD coater, andwherein the VTD coater has heater temperature settings such that aconveyance zone prior to a first distribution zone in which theCdS_(x)Te_(1-x) layer is deposited, relative to a direction of movementof the substrate, has a higher temperature at a center of the conveyancezone and a lower temperature at sides of the conveyance zone. In oneaspect of the method, the VTD coater has heater temperature settingssuch that a pre-heat zone located prior to the conveyance zone, relativeto the direction of movement of the substrate, has a higher temperatureat a center of the pre-heat zone and a lower temperature at sides of thepre-heat zone. In another aspect of the invention, the temperature atthe center of the conveyance zone is about 600° C. and the temperatureat the sides of the conveyance zone is about 510° C., and wherein thetemperature at the center of the pre-heat zone is about 625° C. and thetemperature at the sides of the pre-heat zone is about 535° C. Inanother aspect of the invention, a temperature at sides of the firstdistribution zone is about 640° C. and a temperature at a center of thefirst distribution zone is about 610° C.

In one particular example of the method, the absorber layer is depositedat a second distribution zone, located after the first distributionzone, and wherein a temperature at sides of the second distribution zoneis about 640° C. and a temperature at a center of the seconddistribution zone is about 610° C. In another example, the method caninclude depositing a semiconductor window layer over the TCO stackbefore depositing the CdS_(x)Te_(1-x) layer, wherein the semiconductorwindow layer is deposited in a third distribution zone of the coaterlocated between first zone and the first distribution zone, and whereina temperature of the third distribution zone is about 600° C. In oneparticular aspect of the invention the VTD coater further has heatertemperature settings such that the temperature at the sides of thepre-heat zone is varied, along the direction of movement of thesubstrate. In another aspect of the invention the temperature at thesides of the pre-heat zone is varied, along the direction of movement ofthe substrate, to be about 525° C., about 575° C., about 650° C., andabout 675° C., in order, and the temperature at the center of thepre-heat zone is about 650° C.

The inventive method can include depositing a semiconductor window layerover the TCO stack before depositing the CdS_(x)Te_(1-x) layer, whereinthe semiconductor window layer is deposited in a second distributionzone of the coater located between the pre-heat zone and first zone ofthe coater. In one particular aspect of the invention, the VTD coaterfurther has heater temperature settings such that the temperature at thecenter of the conveyance zone is varied, along the direction of movementof the substrate. On another aspect of the invention the temperature atthe center of the conveyance zone is varied, along the direction ofmovement of the substrate, to be about 560° C., about 590° C., about650° C., and about 600° C., in order, and the temperatures at the sidesof the conveyance zone are about 550° C. In one particular example, atemperature at the second distribution zone is about 600° C. In anotherexample, the temperature at the first distribution zone is about 600° C.In one aspect of the invention, the absorber layer is deposited at athird distribution zone, located after the first distribution zone ofthe coater, and wherein a temperature at sides of the third distributionzone is about 630° C. and a at a center of the third distribution zoneis about 600° C. In another aspect of the invention, the method caninclude a fourth distribution zone, wherein the absorber layer isdeposited at both the third and fourth distribution zones, and wherein atemperature at sides of the fourth distribution zone is about 630° C.and a at a center of the fourth distribution zone is about 600° C.

A method of forming a photovoltaic device according to another exampleof the invention includes forming a non-continuous window layer, formingan absorber layer; and forming a CdS_(x)Te_(1-x) layer between thenon-continuous window layer and the absorber layer, where x is greaterthan 0 and less than 1. In one aspect of the invention, forming anon-continuous window layer includes forming a continuous window andtransforming the continuous window layer into the non-continuous windowlayer. In another aspect of the invention the continuous window layer isformed before forming the CdS_(x)Te_(1-x) layer and the absorber layerand the transforming occurs after forming the CdS_(x)Te_(1-x) layer andthe absorber layer.

In one example of the method, the continuous window layer containssulfur and the transforming the continuous window layer into anon-continuous window layer comprises subjecting the continuous windowlayer to a thermal anneal sufficient to flux sulfur out of the sulfurcontaining continuous window layer and break the continuous window layerinto segments of window layer material. In one aspect of the inventionthe thermal anneal is associated with at least one absorber layerchlorine activation step. In yet another aspect of the invention the atleast one absorber layer chlorine activation step comprises a firstactivation step at a first predetermined temperature and a secondactivation step at a second predetermined temperature.

In one example of the method, the at least one chlorine activation stepis performed in a temperature range of about 350° C. to about 500° C. Inone aspect of the invention, the first and second predeterminedtemperatures are different. In another aspect of the invention the firstchlorine activation step comprises applying CdCl₂ to the absorber layerand annealing at about 400° C. 450° C. for about 15 minutes and thesecond chlorine activation step comprises annealing at about 430° C.450° C. for about 15 minutes.

In one aspect of the invention, the value of x is between about 0.01 andabout 0.3. In another aspect of the invention, the CdS_(x)Te_(1-x) layeris formed to a thickness of about 30 nm to about 1000 nm. In yet anotheraspect of the invention the CdS_(x)Te_(1-x) layer is formed to athickness of about 400 nm to about 600 nm. In another aspect of theinvention the non-continuous window layer comprises segments and has anaverage segment thickness of greater than 0 nm and up to about 50 nm.And in yet another aspect of the invention the absorber layer is formedto a thickness of about 1000 nm to about 8000 nm.

In one particular example of the method, the non-continuous window layercomprises CdS segments having an average thickness of greater than 0 nmto about 50 nm, the CdS_(x)Te_(1-x) layer has a thickness in the rangeof about 30 nm to about 1000 nm, and the absorber layer comprises CdTeand has a thickness in the range of about 1000 nm to about 8000 nm. Inone aspect of the invention, the method of claim 1, further comprisingforming a first electrode and a buffer layer wherein the buffer layer isformed between the first electrode and the non-continuous window layer.In another aspect of the invention, the buffer layer comprises an n-typesemiconductor material. And in yet another aspect of the invention thebuffer layer comprises at least one of a material selected from thegroup consisting of: SnO₂ and ZnO.

In one particular example of the method, the CdS_(x)Te_(1-x) layer isformed having a sulfur concentration gradient over its thickness suchthat the concentration of sulfur is greater at portions closest to thenon-continuous window layer than at portions closest to the absorberlayer. In one aspect of the invention the CdS_(x)Te_(1-x) layer isformed comprising a first sub-layer adjacent the non-continuous windowlayer and a second sub-layer adjacent the first sub-layer. In anotheraspect of the invention the first sub-layer is formed having a highersulfur concentration than the second sub-layer.

In one aspect of the invention at least one of the first and the secondsub-layers are formed having a consistent sulfur content over itsthickness. In another aspect of the invention at least one of the firstsub-layer and the second sub-layer is formed having a graded sulfurcontent over its thickness. In yet another aspect of the invention eachof the first and second sub-layers is formed to a thickness of greaterthan 0 nm and less than or equal to about 500 nm.

Now referring to the accompanying figures, wherein like referencenumbers denote like features, FIG. 1 illustrates one example embodimentof a PV device 100 which includes a semiconductor multi-layer structure122 provided over a transparent conductive oxide (TCO) stack 114 whichis in turn provided over a substrate 102. The substrate 102 provides anouter planar surface which is used to protect the PV device 100 fromenvironmental hazards. Since the substrate 102 receives light incidenton the PV device 100, it should be made of a transparent material. Inthis case, glass or any other suitable material may be used, such asborosilicate glass, soda lime glass, float glass, or a polymer.

TCO stack 114 includes an optional barrier layer 108, TCO layer 110 andoptional buffer layer 112. The TCO stack 114 may be pre-formed onsubstrate. Alternatively, the TCO stack 114 may be formed one layer at atime over substrate 102 during fabrication of device 100. In any case,the barrier layer 108 is used to inhibit sodium diffusion from thesubstrate 102 into other layers (i.e., the window and absorber layers)of the device. Sodium diffusion into these layers may adversely affectdevice efficiency. The barrier layer 108 can be a bi-layer of an SnO₂layer 104 over the substrate 102 and an SiO₂ layer 106 over the SnO₂layer 104 or a single layer of SiO₂ or SnO₂.

The TCO layer 110 functions as one of the two output electrodes of thedevice. Since light has to pass through the TCO layer 110 to reach thesemiconductor layers where it is converted to electricity, it may bemade of a transparent conductive material such as indium tin oxide(ITO), fluorine doped tin oxide (SnO₂:F), or cadmium stannate (Cd₂SnO₄).The buffer layer 112 may be used to reduce electron-hole recombinationat the TCO layer 110 semiconductor multi-layer structure 122 interface.The buffer layer 112 may be made of a metal oxide such as SnO₂, ZnO, ora combination of ZnO and SnO₂. The semiconductor multi-layer 122includes a non-continuous n-type window layer 116, a CdS_(x)Te_(1-x)layer 118, and a p-type absorber layer 120. Those layers are describedin more detail below.

The non-continuous window layer 116 is formed adjacent to TCO stack 114.The non-continuous window layer 116 is preferably formed of CadmiumSulfide. Although the CdS window layer 116 is shown as beingdiscontinuous in FIG. 1 it should be noted that it may be initiallydeposited as a continuous CdS layer and may be made discontinuous duringfurther PV device processing after layers 116, 118 and 120 are formed,as described in detail below.

The layer 118 is formed of CdS_(x)Te_(1-x), where x is greater than zeroand less than one, but is preferably about 0.01 to about 0.3, meaningthat the atomic ratio of sulfur to total anion (i.e., tellurium plussulfur) in the alloy material can be between about 1% to about 30%sulfur with the balance (e.g., about 99% to about 70%) being tellurium.Note, the atomic ratio of concern in the CdS_(x)Te_(1-x) material isthat of the sulfur and tellurium and the cadmium content is notdependent on this atomic ratio, as indicated by the formula above. Therange of sulfur incorporation in the CdS_(x)Te_(1-x) layer 118 is setwith the understanding that increasing the amount of sulfur that can beincorporated into a CdTe material can lower the band gap of the CdTewhile keeping the CdTe p-type, which is important for the p-n junction.Band gap is the energy required to excite electrons from the valenceband to the conduction band, to become a mobile charge carrier anddetermines what portion of the solar spectrum the absorber layer absorbsand photons can be harvested. For example, the optical band gap (E_(g))of CdTe is about 1.55 eV, but this optical band gap is lowered as sulfuris introduced to the CdTe material to form CdS_(x)Te_(1-x). Where x isup to about 0.3, the optical band gap of the absorber layer 120 materialcan be reduced to about 1.4 eV. Lowering the band gap of the CdTe layer120 allows photons of a higher wavelength to be harvested. A point canbe reached when adding sulfur to the CdTe material where the CdTe canbecome n-type, which is undesirable if a p-n junction is to bemaintained for photoconversion. The atomic ratio range of up to about30% sulfur (relative to tellurium plus sulfur) falls safely within anamount of sulfur that can modify the band gap of CdTe, but not convertit to n-type. The CdS_(x)Te_(1-x) layer 118 can also be alloyed or dopedwith other elements, including but not limited to Na, Mg, Zn, Cu, S, Se,Cu, O, and N.

Another preferred value for x is between about 0.01 to about 0.15, suchthat there is between about 1% and about 15% sulfur relative to totalanion and about 99% to about 85% tellurium relative to total anion. Thisrange has been experimentally determined to provide the desired benefitsof the CdS_(x)Te_(1-x) layer 118 (discussed below) without incurringpotential structural defects in the CdS_(x)Te_(1-x) layer 118, which mayoccur due to CdCl₂ activation treatment of the absorber layer 120, suchas physical voids in the CdS_(x)Te_(1-x) layer 118 caused by too muchdiffusion of sulfur therefrom into the absorber layer 120. Anotherpreferred value for x is between about 0.02 to about 0.10, such thatthere is between about 2% and about 10% sulfur relative to total anionand about 98% to about 90% tellurium relative to total anion. This rangeis set based both on the theory that about 5% sulfur in theCdS_(x)Te_(1-x) layer 118 provides the desired benefits of theCdS_(x)Te_(1-x) layer 118 (discussed below) to the PV device and on theexperimental determination that single digit percentages of sulfur inthe CdS_(x)Te_(1-x) layer 118 provided for diffusion to the CdTematerial of the absorber layer 120 are beneficial.

The CdS_(x)Te_(1-x) layer 118 can be between about 20 nm to about 1000nm thick. For example, in a range of about 30 nm to about 1000 nm thick,or in a range of about 50 nm thick to about 500 nm thick. For example,in a range of about 200 nm to about 800 nm thick, about 300 nm to about700 nm thick, about 400 nm to about 600 nm thick.

The CdTe absorber layer 120 is about 500-8000 nm thick, for exampleabout 3300 nm thick. Typically, a PV device would have a continuouswindow layer that is about 75 nm to 200 nm thick. Because the variousdiscontinuous segments of the non-continuous window layer 116 may havedifferent thicknesses, it is convenient to describe the non-continuouswindow layer 116 by an average thickness, which is the average thicknessof each of the segments of the non-continuous window layer 116. Theaverage thickness of the non-continuous window layer in this preferredembodiment can be from greater than 0 nm up to about 200 nm. In oneexample, the average thickness is up to about 100 nm or up to about 50nm.

A back contact layer 124, serving as a second electrode, may be formedover the absorber layer 120. The back contact layer 124 does not have atransparency requirement and thus may be made of a metal such as Mo, Al,Cu, Ag, Au, or a combination thereof. After the formation of the backcontact layer 124, a polymer interlayer 126 may be formed beforeaffixing a back cover 128. The interlayer 126 may be provided over theback contact layer 124 and sides of the layers (114, 122, and 124) ofthe PV device 100. It is used to supplement bonding between thedifferent layers of the device 100 and to inhibit ingress of water orother contaminants into the device. It may be made of a polymer such asethylene-vinyl acetate.

The layers of the PV device 100 may be formed using any known depositiontechnique or combination of techniques. For example, the layers can beformed by chemical vapor deposition (CVD), physical vapor deposition(PVD), chemical bath deposition (CBD), low pressure chemical vapordeposition, atmospheric pressure chemical vapor deposition,plasma-enhanced chemical vapor deposition, thermal chemical vapordeposition, DC or AC sputtering, spin-on deposition, spray-pyrolysis,vapor transport deposition (VTD), closed space sublimation (CSS), assome examples, or a combination thereof. These processes are well knownin the industry and thus will not herein be described in detail.Although certain embodiments are described with respect to certaindeposition techniques, the embodiments are not limited to suchtechniques.

In one example, the CdS window layer 116 is initially formed as acontinuous CdS window layer using vaporized CdS powder, for example in aVTD process. Likewise, CdS_(x)Te_(1-x) layer 118 can be formed over thecontinuous CdS window layer 116 using powdered CdTe as a source which isvaporized and with sulfur vapor being introduced with the CdTe vapor inthe deposition zone to form a layer of CdS_(x)Te_(1-x). The sulfurreacts with the CdTe being deposited to form CdS_(x)Te_(1-x) layer 118.The CdS_(x)Te_(1-x) layer 118 can also be formed using a mixture of CdSand CdTe powders which are vaporized, or using a pre-alloyedCdS_(x)Te_(1-x) material which is vaporized, with preferredcompositional amounts of Cd, S, and Te to provide a CdS_(x)Te_(1-x)target with x in the desired range. The CdTe layer 120 can also beformed over the CdS_(x)Te_(1-x) layer 118 using VTD where powdered CdTeis vaporized and deposited.

After the absorber layer 120 is deposited, a CdCl₂ activation treatmentmay ensue. As noted earlier, during the CdCl₂ activation treatment, thechlorine compound and the associated heat treatment increase sulfurmobility in the window layer 116 and, in the case of the presentembodiment, in the CdS_(x)Te_(1-x) layer 118 as well, causing the sulfurto be more easily fluxed, e.g. sulfur from the window layer dissolvedinto the CdS_(x)Te_(1-x) layer 118 and sulfur from the CdS_(x)Te_(1-x)and window layer dissolved into the absorber layer 120. By providing theCdS_(x)Te_(1-x) layer 118 adjacent to the absorber layer 120, a sourceof sulfur is provided which decreases the sulfur gradient from windowlayer 116 to control the fluxing of the CdS of the window layer 116during the CdCl₂ activation treatment. As a result, it is possible touse more aggressive CdCl₂ activation conditions to achieve an absorberlayer 120 with a larger grain size without uncontrolled and completesulfur consumption of the CdS window layer 116, and thus a destructionof the window layer. During the CdCl₂ activation, the CdS_(x)Te_(1-x)layer 118 allows enough sulfur from the CdS window layer to flux out ofthe window layer to make it discontinuous, but not disappear.

An example of a more aggressive activation treatment which can provide adiscontinuous CdS window layer 116 is provided in the following example.CdCl₂ is applied over the absorber layer 120 as an aqueous solutionhaving a concentration of about 100-600 g/L. Other forms of CdCl₂treatment may also be used, such as CdCl₂ vapor at ambient pressures.The anneal temperature can be up to about 700° C., which is 280-300degrees hotter than typically used. For example, in a range of about200° C. to about 700° C., about 300° C. to about 600° C., about 350° C.to about 500° C., about 400° C. to about 450° C., about 420° C. to about450° C., or up to about 475° C.

The CdCl₂ activation treatment may include a single or a multiple passanneal, meaning more than one heating step. One example of a multiplepass anneal uses a first heating of the CdCl₂ treated absorber layer 120to between about 400° C. to about 450° C., for example about 450° C. forabout 15 minutes and a second heating to between about 430° C. to about450° C., for example about 450° C. for about 15 minutes. A secondexample of a multiple pass anneal uses a first heating to about 450° C.for about 15 minutes and a second heating to about 430° C. for about 20minutes. A third example of a multiple pass anneal uses a first heatingto about 430° C. for about 30 minutes and a second heating to about 430°C. for about 30 minutes. It is possible that, during the second heatingsteps just described, additional CdCl₂ can be applied to the absorberlayer 120, if desired.

The annealing can be continued for as long as is needed to cause the CdSwindow layer to be partially but not completely consumed and form into anon-continuous window layer 116 structure such as that shown in FIG. 1.As examples, the CdCl₂ activation treatment can include an annealing formore than about 10 seconds, for example, in a range of about 10 secondsto about 2 hours, about 10 minutes to about 60 minutes, or about 15minutes to about 30 minutes. The anneal time and temperature of each oreither of the annealing conditions can be adjusted independently ortogether to achieve the non-continuous window layer 116 shown in FIG. 1.Longer anneal times and higher annealing temperatures are required forthicker initial CdS window layers, whereas lower anneal times and loweranneal temperatures can be used for thinner initial CdS window layers.

As described above, the consumption of the continuous CdS window layeris controlled because the sulfur containing CdS_(x)Te_(1-x) layer 118minimizes the sulfur concentration differential, the driving force, forfluxing of sulfur from the window layer 116. While FIG. 1 shows thediscontinuous window layer 116 with segments of a particular shape, thesegments can have any shape, such as semi-spherical, pebble-like, orirregular shapes, among other possible shapes.

The PV device 100 of FIG. 1 with a CdS_(x)Te_(1-x) layer 118 and anon-continuous CdS window layer 116 has been found to have a higherphotoconversion efficiency than a PV device with a continuous CdS windowlayer, or a non-continuous CdS window layer without a CdS_(x)Te_(1-x)layer. Because the gaps in the non-continuous CdS window layer 116 arefilled by the continuous CdS_(x)Te_(1-x) layer 118 which behaves as partof the absorber layer, more photons can reach the absorber layer. Inaddition, the non-continuous CdS window layer 116 has areas where CdS isnot present and is also thinner in average thickness than a conventionalcontinuous window layer, which also allows more photons to pass throughthe non-continuous CdS window layer 116 to the CdS_(x)Te_(1-x) and CdTeabsorber layers 118 and 120.

As noted, the CdS_(x)Te_(1-x) layer 118 is considered to be and behavesas a part of the absorber layer 120, particularly after the mixing ofthe CdS_(x)Te_(1-x) material of the layer 118 and the CdTe material ofthe absorber layer 120 caused by the CdCl₂ activation anneal. It isbelieved that the gaps of non-continuous CdS window layer 116 where noCdS is present do not hinder photoconversion since the desired p-njunction is still provided by the junction between the buffer layer 112(e.g., SnO₂ or ZnO, which are n-type materials) and the CdS_(x)Te_(1-x)layer 118 (which is a p-type material).

Another advantage of providing the CdS_(x)Te_(1-x) layer 118 inassociation with the absorber layer 120 is that the sulfur in theCdS_(x)Te_(1-x) layer 118 can also serve to passivate the CdTe layer 120and the CdS layer 116 surfaces and grain boundaries, respectively, inmuch the same way as Cl atoms from the CdCl₂ activation passivates theCdTe absorber layer. Passivation of the CdTe and CdS layers 120, 116 canreduce defects normally present at the absorber/window interface andthose present at the absorber/CdS_(x)Te_(1-x) interface. With lesselectron loss at grain boundaries and at surface interfaces, the PVdevice 100 photoconversion efficiency can increase.

FIG. 2 shows another example embodiment of a PV device 200 with similarlayers (e.g., 102, 114, 116, 120, 122, 124, 126, 128) to those discussedabove with reference to FIG. 1. These layers can be deposited in thesame manner described above with reference to FIG. 1 and have the samethicknesses. However, FIG. 2 shows an alternative CdS_(x)Te_(1-x) layer118′ to that shown in FIG. 1. Where in FIG. 1 the CdS_(x)Te_(1-x) layer118 has a uniform CdS_(x)Te_(1-x) composition throughout its thickness(x remains constant), in the FIG. 2 embodiment the sulfur concentrationis varied throughout CdS_(x)Te_(1-x) layer 118′. The variedconcentration is a graded composition in which more sulfur is present ata lowermost portion of layer 118′ adjacent the CdS window layer 116 thanat a highermost portion of layer 118′ adjacent the CdTe absorber layer120. In other words, the value x in the formula CdS_(x)Te_(1-x) changesand is larger at the lowermost portion and smaller at the uppermostportion of layer 118′.

FIG. 3 shows another example embodiment of a PV device 300 with similarlayers (e.g., 102, 114, 116, 118, 120, 122, 124, 126, 128) to thosediscussed above with reference to FIG. 1. These layers can be depositedin the same manner described above with reference to FIG. 1 and have thesame thicknesses. However, unlike in FIG. 1, the CdS_(x)Te_(1-x) layer118′ in FIG. 3 is provided as two distinct sub-layers 118 a, 118 b,where the different sub-layers 118 a, 118 b have different sulfur andtellurium material compositions. In this embodiment, CdS_(x)Te_(1-x)layer 118 a has a greater sulfur concentration than CdS_(x)Te_(1-x)layer 118 b, although the concentration of sulfur in one or both of thelayers 118 a and 118 b remains uniform within each layer. In anothervariant, one or both sub-layers 118 a, 118 b may have a varied sulfurconcentration which decreases in the direction from the window layer 116to the absorber layer 120. Thus one of sub-layers 118 a, 118 b may havea graded concentration such that x in the formula CdS_(x)Te_(1-x)changes and is larger at the lowermost portion and smaller at theuppermost portion of the sub-layer, while the other of sub-layers 118 a,11 b may be uniform. Or both of sub-layers 118 a, 118 b may have avaried or uniform sulfur concentration. Each of the sub-layers 118 a,118 b can have a thickness in the range of greater than 0 and less thanor equal to about 500 nm, for example about 250 nm.

As illustrated in the FIGS. 2 and 3 embodiments, the sulfurconcentration in the CdS_(x)Te_(1-x) material can be varied (FIG. 2), oruniform within CdS_(x)Te_(1-x) layers (FIG. 3), or a combination ofuniform and varied as in the FIG. 3 variant, over the thickness of theCdS_(x)Te_(1-x) layer. In both embodiments of FIGS. 2 and 3, the valueof x, and thus sulfur concentration, in the CdS_(x)Te_(1-x) materialbecomes smaller as the CdS_(x)Te_(1-x) layer approaches the absorberlayer 120. In another embodiment, x can change gradually from betweenabout 0.1 to about 0.02 over part of or over the entire thickness of theCdS_(x)Te_(1-x) layer 118 (FIG. 2), or can change stepwise (FIG. 3)between about 0.1 for layer 118 a and about 0.02 for layer 118 b, as oneexample. In another example, x can change between about 0.3 to about0.05 either gradually (FIG. 2) or stepwise (FIG. 3).

An additional advantage of the PV devices 100, 200, 300 of FIGS. 1-3 isthat they reduce the problem of lattice mismatch between the CdS windowand CdTe absorber layer. CdS_(x)Te_(1-x) has a smaller lattice mismatchbetween it and CdS and between it and CdTe than occurs at a CdS/CdTejunction. The graded CdS_(x)Te_(1-x) layer 118′ (FIG. 2.) or thestepwise CdS_(x)Te_(1-x) layers 118 a, 118 b (FIG. 3) further reduce thelattice mismatching between the CdS window layer 116 and the CdTeabsorber layer 120. Since the CST layer 118 contains more sulfur closestto the window layer 116, there is less of a lattice mismatch between itand the window layer 116. Likewise, since the CST layer 118 has more Teclosest to the absorber layer 120, there is less of a lattice mismatchbetween it and the absorber layer 120.

FIG. 4 shows another example embodiment of a PV device 400 with similarlayers (e.g., 102, 114, 116, 118, 120, 122, 124, 126, 128) to thosediscussed above with reference to FIG. 1. However, unlike in FIG. 1, thedeposited CdS_(x)Te_(1-x) layer 118 in FIG. 4 is provided over theabsorber layer 120 and the window layer 116 is continuous as shown.Similarly to the PV device 100 of FIG. 1 discussed above, after thedeposition of semiconductor multi-layer structure 122, a CdCl₂activation treatment may ensue.

The CdS_(x)Te_(1-x) layer 118 of FIG. 4 can be of materials similar to,formed in ways similar to, and have dimensions similar to theCdS_(x)Te_(1-x) layer 118 of FIG. 1. During the formation of theabsorber layer 120, deposition of pure CdTe can be changed to depositionof CdS_(x)Te_(1-x). This can be accomplished by introducing sulfur vaporinto the deposition zone while continuing the deposition of CdTe so thatthe gaseous sulfur and gaseous CdTe react to form CdS_(x)Te_(1-x), asdescribed above in relation to FIG. 1. The sulfur and tellurium contentof the CdS_(x)Te_(1-x) layer 118 of PV device 400 may similarly beuniform as in FIG. 1 or varied as described with respect to FIGS. 2 and3 such that the composition can be varied in a graded or step-wisefashion.

A particular benefit of depositing the CdS_(x)Te_(1-x) layer 118 overthe absorber layer 120, as shown in FIG. 4, is that it may specificallyimprove surface passivation at the absorber's interface with the backcontact layer 124. Improving the absorber/back contact interfaceimproves efficiency by reducing losses at that interface, which mayoccur because of the differences in electrical characteristics of thematerials typically used for the absorber and back contact, e.g., CdTeand metal, respectively. Additionally, depositing the CdS_(x)Te_(1-x)layer 118 over the absorber layer 120 as shown in FIG. 4 is potentiallyeasier to manufacture than depositing the CdS_(x)Te_(1-x) layer 118 inother locations within a PV device because the CdS_(x)Te_(1-x) layer 118can simply be deposited over the absorber layer 120, using similarmaterials and similar deposition techniques as the absorber layer 120.Furthermore, the fluxing of the window layer 116 can be furthercontrolled because the CdS_(x)Te_(1-x) layer 118 is closer to where theCdCl₂ compound is applied thus maintaining a continuous window layer 116even during aggressive CdCl₂ activation treatments.

FIG. 5 shows another example embodiment including a PV device 500 withsimilar layers (e.g., 102, 114, 116, 118, 120, 122, 124, 126, 128) tothose discussed above in reference to FIG. 4; however, theCdS_(x)Te_(1-x) layer 118 is deposited as a layer within the absorberlayer 120, shown as two layers, absorber layers 120 a, 120 b.

The CdS_(x)Te_(1-x) layer 118 of the PV device 500 of FIG. 5 can be ofmaterials similar to, formed in ways similar to, and have dimensionssimilar to the CdS_(x)Te_(1-x) layer 118 of FIG. 1. During the formationof the absorber layer 120 a, deposition of pure CdTe is changed todeposition of CdS_(x)Te_(1-x). This can be accomplished by introducingsulfur vapor into the deposition zone while continuing the deposition ofCdTe so that the gaseous sulfur and gaseous CdTe react to formCdS_(x)Te_(1-x), as described above in relation to FIG. 1. After theCdS_(x)Te_(1-x) layer 118 is formed to the desired thickness, thedeposition process returns to deposition of CdTe without sulfur tocomplete the absorber layer 120 b. The sulfur and tellurium content ofthe CdS_(x)Te_(1-x) layer 118 of PV device 500 may similarly be uniformas in FIG. 1 or varied as described with respect to FIGS. 2 and 3 suchthat the composition can be varied in a graded or step-wise fashion.

Depositing the CdS_(x)Te_(1-x) layer 118 in the location shown in FIG. 5can provide similar advantages to those achieved with the embodimentshown in FIG. 4. An added advantage of this embodiment is that theCdS_(x)Te_(1-x) layer 118 is even more closely associated with theabsorber layers 120 a, 120 b than in the embodiment shown in FIG. 4because it is between the absorber layers 120 a, 120 b and, therefore,potentially can more homogeneously provide sulfur thereto and may moreeffectively control fluxing of sulfur form the window layer andpassivate defects as discussed above.

FIG. 6 shows another example embodiment including a PV device 600 withsimilar layers (e.g., 102, 114, 116, 118, 120, 122, 124, 126, 128) tothose discussed above in reference to FIG. 4; however, twoCdS_(x)Te_(1-x) layers 118 a and 118 b are deposited, with the absorberlayer being over CdS_(x)Te_(1-x) layer 118 a and CdS_(x)Te_(1-x) layer118 b being over absorber layer 120. The CdS_(x)Te_(1-x) layers 118 a,118 b of the PV device 600 of FIG. 6 can be of materials similar to,formed in ways similar to, and have thickness dimensions similar to theCdS_(x)Te_(1-x) layer 118 of FIG. 1. The first CdS_(x)Te_(1-x) layer 118a, deposited between the window layer 116 and the absorber layer 120,functions as a part and an extension of the absorber layer 120. Thesulfur and tellurium content of the CdS_(x)Te_(1-x), layers 118 a,118 bof PV device 600 may similarly be uniform as in FIG. 1 or varied asdescribed with respect to FIGS. 2 and 3 such that the composition can bevaried in a graded or step-wise fashion.

Depositing the CdS_(x)Te_(1-x) layers 118 a, 118 b as shown in FIG. 6can provide the similar sulfur fluxing control and defect passivationadvantages to those achieved with the PV device 400 shown in FIG. 4. Anadded advantage of this embodiment is the potential for more evendistribution of sulfur to the absorber layer 120 from theCdS_(x)Te_(1-x) layers 118 a, 118 b (compared to the embodiment shown inFIG. 4). Also, the proximity of the CdS_(x)Te_(1-x) material to theinterfaces of the absorber layer 120 and each of its adjacent layers,i.e., the window layer 116 and the back contact 124, may more readilypassivate these respective interfaces.

FIG. 7 shows another example embodiment including a PV device 700 withsimilar layers (e.g., 102, 114, 118, 120, 122, 124, 126, 128) to thosediscussed above in reference to FIG. 4, but without a window layer. Inthis embodiment, the CdS_(x)Te_(1-x) layer 118 is deposited between theabsorber layer 120 and the TCO stack 114 while the window layer (e.g.,layer 116 of FIG. 4) is omitted entirely. The CdS_(x)Te_(1-x) layer 118of the PV device 700 of FIG. 7 can be of materials similar to, formed inways similar to, and have thickness dimensions similar to theCdS_(x)Te_(1-x) layer 118 of FIG. 1. The sulfur and tellurium content ofthe CdS_(x)Te_(1-x) layer 118 of PV device 600 may similarly be uniformas in FIG. 1 or varied as described with respect to FIGS. 2 and 3 suchthat the composition can be varied in a graded or step-wise fashion.

In this embodiment, the CdS_(x)Te_(1-x) layer 118 is considered to beand behaves as a part of the absorber layer 120, particularly after thefluxing of material caused by the CdCl₂ activation treatment. It isbelieved that a CdS window layer (e.g., window layer 116) is notnecessary for photoconversion in the current embodiment. Without the CdSmaterial, the desired p-n junction is provided by the junction betweenthe buffer layer 112 (e.g., SnO₂ or ZnO, which are n-type materials) andthe CdS_(x)Te_(1-x), layer 118 (which is p-type material).

Depositing the CdS_(x)Te_(1-x) layer 118 in the location shown in FIG. 7can provide similar advantages to those achieved with the examplestructure shown in FIG. 1. Replacing the window layer 116 with theCdS_(x)Te_(1-x) layer 118 also further improves the PV device efficiencybecause the CdS material typically used for the window layer 116 isrelatively light absorbent, thus more photons can be absorbed by theCdS_(x)Te_(1-x) layer 118 and the absorber layer 120 forphotoconversion. In addition to these advantages, omitting a separatewindow layer 116 can reduce the steps needed in manufacturing the PVdevice 700 as well as the materials needed to produce the PV device 700as compared to other disclosed embodiments.

FIG. 7A shows an additional example embodiment including a PV device700A with similar layers (e.g., 102, 114, 118, 120, 122, 124, 126, 128)to those discussed above in reference to FIG. 7, also without a windowlayer, but including an additional sulfur-containing layer 119 betweenthe CdS_(x)Te_(1-x) layer 118 and the absorber layer 120. The additionalsulfur-containing layer 119 of FIG. 7A may comprise a CdS_(x2)Te_(1-x2)material (having a lower sulfur content than CdS_(x)Te_(1-x) layer 118(e.g., x2<x)), CdS, ZnS, ZnO_(x)S_(y), or any combination thereof.

As in the embodiment of FIG. 7, the CdS_(x)Te_(1-x) layer 118 of FIG. 7Ais deposited between the absorber layer 120 and the TCO stack 114 whilethe window layer (e.g., layer 116 of FIG. 4) is omitted entirely. TheCdS_(x)Te_(1-x) layer 118 of the PV device 700A can be of materialssimilar to, formed in ways similar to, and have thickness dimensionssimilar to the CdS_(x)Te_(1-x) layer 118 of FIG. 7. Alternatively, inthe PV device 700A, the CdS_(x)Te_(1-x) layer 118 includes values of xfrom about 0.05 to about 0.5, meaning that the atomic ratio of sulfur tototal anion in the alloy material can be between about 5% to about 50%sulfur with the balance being tellurium. The sulfur and telluriumcontent of the CdS_(x)Te_(1-x) layer 118 of PV device 700A may similarlybe uniform as in FIG. 1 or varied as described with respect to FIGS. 2and 3 such that the composition can be varied in a graded or step-wisefashion.

In addition to the advantages discussed with respect to FIG. 7 of thedeposited CdS_(x)Te_(1-x) layer 118, depositing the additionalsulfur-containing layer 119 as shown in FIG. 7A allows for bettercontrol of sulfur fluxing and maintenance of the desired structure ofCdS_(x)Te_(1-x) layer 118 while also targeting defect passivationthroughout the semiconductor multi-layer 122.

FIG. 7B shows an additional example embodiment including a PV device700B with similar layers (e.g., 102, 114, 118, 120, 122, 124, 126, 128)to those discussed above in reference to FIG. 7, also without a windowlayer, but including an additional sulfur-containing layer 119 withinthe absorber layer 120 (shown as two absorber sub-layers 120 a, 120 b).The additional sulfur-containing layer 119 of FIG. 7B may comprise aCdS_(x2)Te_(1-x2) material (having a lower sulfur content thanCdS_(x)Te_(1-x) layer 118 (e.g., x2<x)), CdS, ZnS, ZnO_(x)S_(y), or anycombination thereof.

As in the embodiment of FIG. 7, the CdS_(x)Te_(1-x) layer 118 of FIG. 7Bis deposited between the absorber layer 120 and the TCO stack 114 whilethe window layer (e.g., layer 116 of FIG. 4) is omitted entirely. TheCdS_(x)Te_(1-x) layer 118 of the PV device 700B can be of materialssimilar to, formed in ways similar to, and have thickness dimensionssimilar to the CdS_(x)Te_(1-x) layer 118 of FIG. 7. Alternatively, inthe PV device 700B, the CdS_(x)Te_(1-x) layer 118 includes values of xfrom about 0.05 to about 0.5, meaning that the atomic ratio of sulfur tototal anion in the alloy material can be between about 5% to about 50%sulfur with the balance being tellurium. The sulfur and telluriumcontent of the CdS_(x)Te_(1-x) layer 118 of PV device 700B may similarlybe uniform as in FIG. 1 or varied as described with respect to FIGS. 2and 3 such that the composition can be varied in a graded or step-wisefashion.

In addition to the advantages discussed with respect to FIG. 7 of thedeposited CdS_(x)Te_(1-x) layer 118, depositing the additionalsulfur-containing layer 119 as shown in FIG. 7B allows for bettercontrol of sulfur fluxing and maintenance of the desired structure ofCdS_(x)Te_(1-x) layer 118 while also targeting defect passivationthroughout the semiconductor multi-layer 122.

FIG. 7C shows an additional example embodiment including a PV device700C with similar layers (e.g., 102, 114, 118, 120, 122, 124, 126, 128)to those discussed above in reference to FIG. 7, also without a windowlayer, but including an additional sulfur-containing layer 119 over theabsorber layer 120. The additional sulfur-containing layer 119 of FIG.7C may comprise a CdS_(x2)Te_(1-x2) material (having a lower sulfurcontent than CdS_(x)Te_(1-x) layer 118 (e.g., x2<x)), CdS, ZnS,ZnO_(x)S_(y), or any combination thereof.

As in the embodiment of FIG. 7, the CdS_(x)Te_(1-x) layer 118 of FIG. 7Cis deposited between the absorber layer 120 and the TCO stack 114 whilethe window layer (e.g., layer 116 of FIG. 4) is omitted entirely. TheCdS_(x)Te_(1-x) layer 118 of the PV device 700C can be of materialssimilar to, formed in ways similar to, and have thickness dimensionssimilar to the CdS_(x)Te_(1-x) layer 118 of FIG. 7. Alternatively, inthe PV device 700C, the CdS_(x)Te_(1-x) layer 118 includes values of xfrom about 0.05 to about 0.5, meaning that the atomic ratio of sulfur tototal anion in the alloy material can be between about 5% to about 50%sulfur with the balance being tellurium. The sulfur and telluriumcontent of the CdS_(x)Te_(1-x) layer 118 of PV device 700C may similarlybe uniform as in FIG. 1 or varied as described with respect to FIGS. 2and 3 such that the composition can be varied in a graded or step-wisefashion.

In addition to the advantages discussed with respect to FIG. 7 of thedeposited CdS_(x)Te_(1-x) layer 118, depositing the additionalsulfur-containing layer 119 as shown in FIG. 7C allows for bettercontrol of sulfur fluxing and maintenance of the desired structure ofCdS_(x)Te_(1-x) layer 118 while also targeting defect passivationthroughout the semiconductor multi-layer 122.

FIG. 7D shows an additional example embodiment including a PV device700D with similar layers (e.g., 102, 114, 118, 120, 122, 124, 126, 128)to those discussed above in reference to FIG. 7, also without a windowlayer, but including a first additional sulfur-containing layer 119 abetween the CdS_(x)Te_(1-x) layer 118 and the absorber layer 120 (shownas two absorber sub-layers 120 a, 120 b) and a second additionalsulfur-containing layer 119 b within the absorber layer 120 (i.e.between absorber sub-layers 120 a, 120 b). Each of the additionalsulfur-containing layers 119 a, b of FIG. 7D may comprise aCdS_(x2)Te_(1-x2) material (having a lower sulfur content thanCdS_(x)Te_(1-x) layer 118 (e.g., x2<x)), CdS, ZnS, ZnO_(x)S_(y), or anycombination thereof.

As in the embodiment of FIG. 7, the CdS_(x)Te_(1-x) layer 118 of FIG. 7Dis deposited between the absorber layer 120 and the TCO stack 114 whilethe window layer (e.g., layer 116 of FIG. 4) is omitted entirely. TheCdS_(x)Te_(1-x) layer 118 of the PV device 700D can be of materialssimilar to, formed in ways similar to, and have thickness dimensionssimilar to the CdS_(x)Te_(1-x) layer 118 of FIG. 7. Alternatively, inthe PV device 700D, the CdS_(x)Te_(1-x) layer 118 includes values of xfrom about 0.05 to about 0.5, meaning that the atomic ratio of sulfur tototal anion in the alloy material can be between about 5% to about 50%sulfur with the balance being tellurium. The sulfur and telluriumcontent of the CdS_(x)Te_(1-x) layer 118 of PV device 700D may similarlybe uniform as in FIG. 1 or varied as described with respect to FIGS. 2and 3 such that the composition can be varied in a graded or step-wisefashion.

In addition to the advantages discussed with respect to FIG. 7 of thedeposited CdS_(x)Te_(1-x) layer 118, depositing the additionalsulfur-containing layers 119 a,b as shown in FIG. 7D allows for bettercontrol of sulfur fluxing and maintenance of the desired structure ofCdS_(x)Te_(1-x) layer 118 while also targeting defect passivationthroughout the semiconductor multi-layer 122.

FIG. 7E shows an additional example embodiment including a PV device700E with similar layers (e.g., 102, 114, 118, 120, 122, 124, 126, 128)to those discussed above in reference to FIG. 7, also without a windowlayer, but including a first additional sulfur-containing layer 119 abetween the CdS_(x)Te_(1-x) layer 118 and the absorber layer 120 and asecond additional sulfur-containing layer 119 b over the absorber layer120. Each of the additional sulfur-containing layers 119 a,b of FIG. 7Emay comprise a CdS_(x2)Te_(1-x2) material (having a lower sulfur contentthan CdS_(x)Te_(1-x) layer 118 (e.g., x2<x)), CdS, ZnS, ZnO_(x)S_(y), orany combination thereof.

As in the embodiment of FIG. 7, the CdS_(x)Te_(1-x) layer 118 of FIG. 7Eis deposited between the absorber layer 120 and the TCO stack 114 whilethe window layer (e.g., layer 116 of FIG. 4) is omitted entirely. TheCdS_(x)Te_(1-x) layer 118 of the PV device 700E can be of materialssimilar to, formed in ways similar to, and have thickness dimensionssimilar to the CdS_(x)Te_(1-x) layer 118 of FIG. 7. Alternatively, inthe PV device 700E, the CdS_(x)Te_(1-x) layer 118 includes values of xfrom about 0.05 to about 0.5, meaning that the atomic ratio of sulfur tototal anion in the alloy material can be between about 5% to about 50%sulfur with the balance being tellurium. The sulfur and telluriumcontent of the CdS_(x)Te_(1-x) layer 118 of PV device 700E may similarlybe uniform as in FIG. 1 or varied as described with respect to FIGS. 2and 3 such that the composition can be varied in a graded or step-wisefashion.

In addition to the advantages discussed with respect to FIG. 7 of thedeposited CdS_(x)Te_(1-x) layer 118, depositing the additionalsulfur-containing layers 119 a,b as shown in FIG. 7E allows for bettercontrol of sulfur fluxing and maintenance of the desired structure ofCdS_(x)Te_(1-x) layer 118 while also targeting defect passivationthroughout the semiconductor multi-layer 122.

FIG. 8 shows another example embodiment including a PV device 800 withsimilar layers (e.g., 102, 114, 118, 124, 126, 128) to those discussedabove in reference to FIG. 4; however, this embodiment omits both thewindow layer and the absorber layer. In this example embodiment, theCdS_(x)Te_(1-x) layer 118 is deposited between the TCO stack 114 and theback contact layer 124 and replaces both the window layer and absorberlayer (e.g., layers 116, 120 of FIG. 4). In other words, theCdS_(x)Te_(1-x) layer 118 itself serves the function of an absorber forthe PV device 800 without the need for a separate, dedicated absorberlayer, e.g., layer 120 of FIG. 4. Without the CdS/CdTe junction (of theomitted window layer and absorber layer), the desired p-n junction isprovided by the junction between the buffer layer 108 (e.g., SnO₂ orZnO, which are n-type materials) and the CdS_(x)Te_(1-x) layer 118(which is p-type material).

The CdS_(x)Te_(1-x) layer 118 of the PV device 800 of FIG. 8 can be ofmaterials similar to and formed in ways similar to the CdS_(x)Te_(1-x)layer 118 of FIG. 1. However, in this embodiment, the CdS_(x)Te_(1-x)layer 118 has a greater thickness than in other embodiments in order tofunction as an absorber layer. For example, the CdS_(x)Te_(1-x) layer118 of the PV device 800 can be about 2 μm to about 8 μm thick. Anabsorber thickness of about 3.3 μm is advantageous to effectively absorbthe visible spectrum of light (a minimum 2 μm thickness is needed tofully absorb visible light) and also separate the absorption depth ofthe light within the layer from the associated back contact layer 118 sothat photoconversion does not interfere with current at theabsorber/back contact interface. Such interference can occur if thelocation of light absorption is too close to the absorber layer'sinterface with the back contact.

Furthermore, the CdS_(x)Te_(1-x) layer 118 shown in FIG. 8 can have asingle, fixed value for x, so that the CdS_(x)Te_(1-x) composition issubstantially maintained over its thickness. Alternatively, theCdS_(x)Te_(1-x) layer 118 can be compositionally graded from front toback, as in the CdS_(x)Te_(1-x) layers 118 of FIGS. 2 and 3 such thatthe portions of the CdS_(x)Te_(1-x) layer 118 nearer the TCO stack 110include a greater proportional amount of sulfur and the portions of theCdS_(x)Te_(1-x) layer 118 nearer the back contact layer 118 include agreater proportional amount of tellurium. It is possible that x canchange in this way from between about 1 to about 0 over part of or theentire thickness of the CdS_(x)Te_(1-x) layer 118 of the PV device 800of FIG. 8. The transition from x being about 1 to x being about 0 can begradual over the thickness of the CdS_(x)Te_(1-x) layer 118, as in thePV device 200 of FIG. 2. Alternatively, the transition can be staggeredor abruptly change, as in a step-wise fashion, from the CdS_(x)Te_(1-x)layer 118 being predominantly CdS to predominantly CdTe, as in the PVdevice 300 of FIG. 3.

In addition to the advantages discussed with respect to FIGS. 1-7,depositing the CdS_(x)Te_(1-x) layer 118 as it is shown in FIG. 8 andomitting a separate window layer (e.g., layer 116) and absorber layer(e.g., layer 120) can reduce the steps needed in manufacturing the PVdevice 800 as well as the materials needed to produce the PV device 800.

The CdS_(x)Te_(1-x) layer 118 of each of the disclosed embodiments mayalso be doped with copper. Doping with copper provides for increasedp-type doping of the CdS_(x)Te_(1-x) layer 118 and better allows forintermixing of the CdS_(x)Te_(1-x) layer 118 and absorber layer 120. Thecopper-doped CdS_(x)Te_(1-x) layer 118 may be formed, for example, usinga vapor transport deposition (VTD) process, similar to that describedabove. Copper and sulfur are co-deposited into a CdTe layer during theVTD process. A copper source powder (such as, e.g., CuCl, CuCl₂, orCu₃N) and a CdS powder are mixed with CdTe powder, in the desired Cu/Sconcentrations, to form a powder blend. As one example, in the powderblend, the copper source powder may have a concentration of 100-5000 ppmand the CdS may have a concentration of 0.1-20%. The powder blend isthen vaporized at an elevated temperature and then condensed on thesubstrate, thus forming the copper-doped CdS_(x)Te_(1-x) layer thereon.The as-deposited copper-doped CdS_(x)Te_(1-x), layer may include copperin the range of 1-50 ppm. Copper may also be introduced via a secondaryvaporizer to avoid the reaction with tellurium which forms Cu—Tecompounds which have low volatility.

A general discussion of a method of deposition of the CdS_(x)Te_(1-x)layer 118 of the disclosed embodiments is provided above with respect tothe example embodiment of FIG. 1. These described techniques can be usedto form any of the embodiments disclosed in FIGS. 1-8. However, it hasbeen determined that the temperature of the TCO-coated substrate (ontowhich the CdS_(x)Te_(1-x) layer 118, and the semiconductor window 116and absorber 120 layers (where applicable) are deposited) during VTDdeposition of CdS_(x)Te_(1-x) layer 118 can have a significant impact onthe grain size, packing density of the grains, adhesion, and filmcoverage on the plate. A deposited CdS_(x)Te_(1-x) layer 118 should havelarge grain size, high packing density and cover the entire plate. Thetemperature of the TCO-coated substrate during deposition of theCdS_(x)Te_(1-x) layer 118 can be controlled by adjusting the settings ofthe heaters in various transport sections and distribution zones of theVTD coater. Several example embodiments of controlling these temperaturesettings are now described with respect to FIG. 9.

FIG. 9 shows a top down view of an example embodiment of a VTD coater900 with the arrow “A” indicating a direction of conveyance of asubstrate to be coated through the illustrated zones. Zones 910, 915 and920 of coater 900 are conveyance zones, whereas distribution zones 901,902, 903, 904 are zones at which material may be deposited onto asubstrate, according to known VTD techniques, as discussed above.Depending on the particular process configuration, zones 910, 915 can beused for pre-heating the substrate and zone 920 can be heated to preventthe coated substrate from cooling too quickly after deposition.Distribution zones 901, 902, 903, 904 may be used for depositing amaterial but a material is not necessarily deposited at each of thesezones, depending on the process configurations. Specific examples arediscussed below. In the VTD coater 900, various heater settings may beused in different transverse sections of each of zones 910, 915, 920 ofthe coater 900. For example, each of sections 910 a (side), 910 b(center) and 910 c (side) of zone 910 may have different heatersettings. This is also true for sections 915 a (side), 915 b (center)and 915 c (side) of zone 915, and sections 920 a (side), 920 b (center)and 920 c (side) of zone 920. Similarly, each of distribution zones 901,902, 903, 904 may use various heater settings at a center (b) and sides(a, c) thereof.

It has been discovered that control of the heater temperature settingsas the substrate passes through the VTD coater 900 can be used toprovide PV devices with improved efficiency as well as high open circuitvoltage (“Voc”) and high stability. For example, heater settings can beused to control CdS_(x)Te_(1-x) film coverage on the plate (e.g., byusing lower side temperatures than a center temperature in zone 915,just prior to CdS_(x)Te_(1-x) deposition), reduce/eliminate “hopping” ofthe plate (e.g., when a cold plate enters zone 910, edges will becomehot faster than the center; the temperature profiles of the disclosedembodiments, discussed below, in zone 910 reduce/eliminate thisproblem), and/or to provide a CdS_(x)Te_(1-x) layer 118 with large grainsize. The VTD coater 900 shown in FIG. 9 can be used for depositingmaterial in any of the previously described embodiments, andadditionally for structures including a semiconductor window layer, aCdS_(x)Te_(1-x) layer and a semiconductor absorber layer deposited overa TCO stack.

One non-limiting example of particular heater settings for use in coater900 is now described. In this example embodiment, coater 900 can be usedfor forming a PV module which includes CdS_(x)Te_(1-x) layer 118,absorber layer 120 and no window layer (e.g., the embodiment of FIG. 4).In this example, a substrate 102, onto which a TCO stack 114 has alreadybeen deposited, enters the coater 900 at zone 910 and, traveling in thedirection of arrow “A”, passes through distribution zone 901, zone 915and distribution zone 902. In this embodiment, no material is depositeduntil the substrate reaches distribution zone 903; thus, zones 910, 915and distribution zones 901, 902 act to pre-heat the substrate prior todistribution zone 903. A CdS_(x)Te_(1-x) layer 118 is deposited atdistribution zone 903 and a CdTe absorber layer 120 is deposited atdistribution zone 904. Zone 920 is a post-deposition zone of the coater900.

In this example embodiment, the temperature in sections 910 a, 910 c canbe about 535° C. and in section 910 b can be about 625° C. Thetemperature in sections 915 a, 915 c can be about 510° C. and in section915 b can be about 600° C. Distribution zones 901 and 902 are kept at atemperature of about 600° C., across all sections 901 a, 901 b, 901 c,902 a, 902 b, 902 c thereof. In each of zones 903, 904, whereCdS_(x)Te_(1-x) layer 118 and absorber layer 120, respectively, aredeposited, heaters in outside sections (903 a, 903 c, 904 a, 904 c) maybe set to about 940° C. and in the center section (903 b, 904 b) may beset to about 610° C. The temperature in sections 920 a, 920 c can beabout 630° C. and in section 920 b can be about 610° C., to prevent thecoated module from cooling too quickly. In this example, CdS_(x)Te_(1-x)layer 118 may be deposited using mixed CdTe—CdS or an alloy powder witha CdS concentration of about 25 mole %. It has been discovered thatusing the relatively lower side temperature heater settings in the zone915 of coater 900 (e.g., the temperature in sections 915 a, 915 c islower than the temperature in section 915 b), as in this example, helpsensure full coverage of the CdS_(x)Te_(1-x) film 118 on the plate.Further, using relatively lower side temperature heater settings in thezone 910 of coater 900 (e.g., the temperature in sections 910 a, 910 cis lower than the temperature in section 910 b) helps prevent platehopping.

Another non-limiting example embodiment of particular heater settingsfor use in coater 900 is now described. In this embodiment, coater 900can be used for forming a PV module which includes, among other layers,a semiconductor window layer 116, a CdS_(x)Te_(1-x) layer 118 and asemiconductor absorber layer 120, e.g. the embodiments of FIGS. 1-3 and6, as well as other embodiments. In this example, a substrate 102, ontowhich a TCO 114 stack has already been deposited, enters the coater 900at zone 910, traveling in the direction of arrow “A”, and passes throughdistribution zone 901 and zone 915. In this embodiment, no material isdeposited until the substrate reaches distribution zone 902; thus, zones910, 915 and distribution zone 901 act to pre-heat the substrate priorto distribution zone 902. A CdS semiconductor window layer 116 isdeposited at distribution zone 902, a CdS_(x)Te_(1-x) layer 118 isdeposited at distribution zone 903 and a CdTe absorber layer 120 isdeposited at distribution zone 904. Zone 920 is a post-deposition zoneof the coater. Additional layers may also be deposited and the order oflayer deposition may be altered to form the other embodiments disclosedherein.

In this embodiment, the temperature in sections 910 a, 910 c can beabout 535° C. and in section 910 b can be about 625° C. The temperaturein sections 915 a, 915 c can be about 510° C. and in section 915 b canbe about 600° C. Distribution zone 901 is kept at a temperature of about600° C., across all sections 901 a, 901 b, 901 c, thereof. Distributionzone 902, in which semiconductor window layer 116 is deposited, is keptat a temperature of about 600° C., across all sections 902 a, 902 b, 902c, thereof. In each of distribution zones 903 and 904, whereCdS_(x)Te_(1-x) layer 118 and absorber layer 120, respectively, aredeposited, heaters in outside sections (903 a, 903 c, 904 a, 904 c) maybe set to about 640° C. and in the center section (903 b, 904 b) may beset to about 610° C. The temperature in post-deposition sections 920 a,920 c can be about 630° C. and in section 920 b can be about 610° C., toprevent the coated module from cooling too quickly. In this example,CdS_(x)Te_(1-x) layer 118 may be deposited from a CdS_(x)Te_(1-x) powderfeed with high CdS concentration, up to 90 mole %. Increased sulfurconcentration in the CdS_(x)Te_(1-x) powder produces PV modules havinghigher stability, and “wakeup” (wakeup refers to when the module beginsgenerating current, for example, in the morning after not being exposedto sunlight all night) at high temperatures, which indicates the moduleswould be appropriate for operation in hot climates. As indicated above,using relatively lower side temperature heater settings in the zone 915of coater 900 (e.g., the temperature in sections 915 a, 915 c is lowerthan the temperature in section 915 b) helps ensure full coverage of theCdS_(x)Te_(1-x) film 118 on the plate. Further, using relatively lowerside temperature heater settings in the zone 910 of coater 900 (e.g.,the temperature in sections 910 a, 910 c is lower than the temperaturein section 910 b) helps prevent plate hopping.

Another non-limiting example embodiment of particular heater settingsfor use in coater 900 is now described. In this embodiment, coater 900can be used for forming a PV module which includes, among other layers,a semiconductor window layer 116, a CdS_(x)Te_(1-x) layer 118 and asemiconductor absorber layer 120, e.g. the embodiments of FIGS. 1-3 and6, as well as other embodiments. In this example, a substrate 102, ontowhich a TCO stack 114 has already been deposited, enters the coater 900at zone 910 and travels in the direction of arrow “A”. A CdSsemiconductor window layer 116 is deposited at distribution zone 901.Zone 910 acts to pre-heat the substrate prior to deposition of thesemiconductor window layer 116 in distribution zone 901. The substratecontinues through zone 915. A CdS_(x)Te_(1-x) layer 118 is deposited atzone 902 and a CdTe absorber layer 120 is deposited (in two parts) atzones 903, 904. Zone 920 is a post-deposition zone of the coater 900.This configuration, including the two part deposition of the CdTeabsorber layer, allows for faster line speeds for production. Additionallayers may also be deposited and the order of layer deposition may bealtered to form the other embodiments disclosed herein.

In this embodiment, the temperature settings of the heaters in sections910 a and 910 c increase as the plate moves (in the direction of arrow“A”) closer to distribution zone 901. For example, the heaters insections 910 a and 910 c may have four different temperature settings,increasing in the direction of arrow “A” from about 525° C. to about575° C. to about 650° C. to about 675° C., with center section 910 bkept at a constant temperature of about 650° C. Combining thistemperature ramp-up with the relatively lower side temperatures inpre-heating zone 910 even further reduces the problem of plate hopping,as compared to the embodiment using only the side/center temperaturedifferential. In other embodiments, the temperature in zone 9101) mayalso be increased in the direction of arrow “A” rather than being keptat a constant temperature. Distribution zone 901, in which semiconductorwindow layer 116 is deposited, is kept at a temperature of about 600°C., across all sections 901 a, 901 b, 901 c, thereof. In zone 915, thetemperature settings of the heaters in section 915 b change as the platemoves (in the direction of arrow “A”) closer to zone 902, similar to thetemperature change in section 910 a, 910 c of zone 910. For example,section 915 b may have four different temperature settings, increasingin the direction of arrow “A” from about 560° C. to about 590° C. toabout 650° C., and then decreasing back to about 600° C., with sidesections 915 a, 915 c kept at a constant temperature of about 550° C.This temperature configuration in zone 915 helps ensure both fullcoverage of the CdS_(x)Te_(1-x) layer 118 on the substrate (since thesides are maintained at relatively lower temperatures than the center)and prevents unwanted re-sublimation of the deposited CdS layer 116prior to deposition of the CST layer 118 (by maintaining an appropriatetemperature). Distribution zone 902, in which CdS_(x)Te_(1-x) layer 118is deposited, is kept at a temperature of 600° C., across all sections902 a, 902 b, 902 c, thereof. In each of distribution zones 903, 904,where the absorber layer 120 is deposited, heaters in outside sections(903 a, 903 c, 904 a, 904 c) may be set to about 630° C. and in thecenter section (903 b, 904 b) may be set to about 600° C. In thisexample, CdS_(x)Te_(1-x) layer 118 may be deposited from aCdS_(x)Te_(1-x) powder feed with CdS concentration of about 5 mole %.The temperature in post-deposition sections 920 a, 920 c can be about630° C. and in section 920 b can be about 610° C., to prevent the coatedmodule from cooling too quickly. Modules produced by a VTD coater usingthe temperature settings of this example embodiment yielded anefficiency enhancement of about 0.25%.

It should be noted that the specific temperatures described with respectto the embodiments of FIG. 9 are merely provided as examples. Other zonetemperature settings, may be used, for example at varying transportspeeds; however the zones should have similar relative zone temperaturesas those described above in order to achieve the beneficial effects.

As discussed above, the PV modules 100, 200, 300, 400, 500, 600, 700,700A-E, and 800 of any of the disclosed embodiments of FIGS. 1-8 can befabricated beginning with a substrate layer 102, forming subsequentlayers, e.g., 108, 110, 112, 116, 118, 120, 124, as relevant to thespecific embodiment, in sequence over the substrate layer 102, or can beprocessed in a reverse sequence, beginning with a back cover 128. Also,the PV modules 100, 200, 300, 400, 500, 600, 700, 700A-E, and 800 can bepartially pre-fabricated such that a substrate and front contact, e.g.,layers 102 and 114, can be provided as a pre-fabricated and theremaining layers subsequently provided thereover. In addition, somelayers illustrated may be omitted and the interlayer 126 may be providedonly on the sides of the other material layers and not between the backcontact 124 and the back cover 128.

The aforementioned advantages relating to the provision of theCdS_(x)Te_(1-x) layer 118 in the PV devices 100, 200, 300, 400, 500,600, 700, 700A-E, and 800 are examples and non-limiting. Otheradvantages may be realized and the invention should not be limited to orby those discussed above.

Each layer described herein may include more than one layer or film.Additionally, each layer can cover all or a portion of the device and/orall or a portion of the underlying material. For example, a “layer” caninclude any amount of any material that contacts all or a portion of asurface. Further, it is also possible for distinct boundaries betweendisclosed layers to be lost during subsequent manufacturing steps. Forexample, during the CdCl₂ activation step, it is possible for part ofthe CST layer 118 material to diffuse into the absorber layer 120. Thisphenomenon, or intermingling, can cause distinct boundaries betweenlayers 118 and 120, for example, to be blended such that the layers maynot be separately discernible even when using Scanning or Transmissionelectron microscopy (SEM/TEM) imaging.

Although a number of embodiments have been described, it will beunderstood that various modifications can be made without departing fromthe scope of the invention. Also, it should also be understood that theappended drawings are not necessarily to scale, presenting a somewhatsimplified representation of various features and basic principles ofthe invention. The invention is not intended to be limited by anyportion of the disclosure and is defined only by the appended claims.

What is claimed is:
 1. A photovoltaic device, comprising: an absorberlayer; a non-continuous window layer; and a CdS_(x)Te_(1-x) layerbetween the non-continuous window layer and the absorber layer, where xis greater than 0 and less than
 1. 2. The photovoltaic device of claim1, wherein x in the CdSxTe_(1-x) is between about 0.01 and about 0.3. 3.The photovoltaic device of claim 1, wherein the non-continuous windowlayer comprises discontinuous segments of CdS material.
 4. Thephotovoltaic device of claim 3, wherein the absorber layer comprisesCdTe.
 5. The photovoltaic device of claim 3, wherein the non-continuouswindow layer has an average segment thickness of greater than 0 nm andup to about 50 nm.
 6. The photovoltaic device of claim 5, wherein theCdS_(x)Te_(1-x) layer has a thickness of about 30 nm to about 1000 nm.7. The photovoltaic device of claim 6, wherein the absorber layer has athickness from about 1000 nm to about 8000 nm.
 8. The photovoltaicdevice of claim 1, wherein the non-continuous window layer comprises CdSsegments which have an average thickness of less than about 50 nm, theCdS_(x)Te_(1-x) layer has a thickness in the range of about 30 nm toabout 1000 nm, and the absorber layer comprises CdTe and has a thicknessin the range of about 1000 nm to about 8000 nm.
 9. The photovoltaicdevice of claim 1, further comprising a first electrode and a bufferlayer between the first electrode and the non-continuous window layer.10. The photovoltaic device of claim 9, wherein the buffer layercomprises an n-type semiconductor material.
 11. The photovoltaic deviceof claim 1, wherein the CdS_(x)Te_(1-x) layer has a sulfur concentrationgradient over its thickness such that a concentration of sulfur isgreater at portions closest to the non-continuous window layer than atportions closest to the absorber layer.
 12. The photovoltaic device ofclaim 11, wherein the CdS_(x)Te_(1-x) layer comprises a first sub-layeradjacent the non-continuous window layer and a second sub-layer adjacentthe first sub-layer.
 13. The photovoltaic device of claim 12, whereinthe first sub-layer has a higher sulfur concentration than the secondsub-layer.
 14. The photovoltaic device of claim 12, wherein at least oneof the first and the second sub-layers has a consistent sulfur contentover its thickness.
 15. The photovoltaic device of claim 12, wherein atleast one of the first sub-layer and the second sub-layer has a gradedsulfur content over its thickness.
 16. A photovoltaic device,comprising: a first semiconductor absorber layer; a semiconductor windowlayer on a first side of the semiconductor absorber layer; and a firstCdS_(x)Te_(1-x) layer on a second side of the semiconductor absorberlayer, wherein x is greater than 0 and less than
 1. 17. The photovoltaicdevice of claim 16, wherein x is between about 0.01 to about 0.30. 18.The photovoltaic device of claim 16, wherein the CdS_(x)Te_(1-x) layeris about 50 nm to about 1000 nm thick.
 19. The photovoltaic device ofclaim 16, wherein the semiconductor absorber layer comprises sulfurprovided by the CdS_(x)Te_(1-x) layer.
 20. The photovoltaic device ofclaim 16, wherein the CdS_(x)Te_(1-x) layer is a copper-dopedCdS_(x)Te_(1-x) layer.
 21. The photovoltaic device of claim 16 furthercomprising a second semiconductor absorber layer on a side of the firstCdS_(x)Te_(1-x) layer opposite the first semiconductor absorber layer22. The photovoltaic device of claim 21, wherein at least one of thefirst and the second semiconductor absorber layers comprise sulfurprovided by the first CdS_(x)Te_(1-x) layer.
 23. The photovoltaic deviceof claim 18 further comprising a second CdS_(x)Te_(1-x) layer betweenthe semiconductor window layer and the first semiconductor absorberlayer.
 24. The photovoltaic device of claim 1, wherein at least one ofthe first CdS_(x)Te_(1-x) layer and the second CdS_(x)Te_(1-x) layer isabout 50 nm to about 1000 nm thick.
 25. The photovoltaic device of claim1, wherein the semiconductor absorber layer comprises sulfur provided byat least one of the first CdS_(x)Te_(1-x) layer and the secondCdS_(x)Te_(1-x) layer.
 26. A photovoltaic device, comprising: a bufferlayer over a front contact; and a CdS_(x)Te_(1-x) layer between thebuffer layer and a back contact, wherein x is greater than 0 and lessthan 1, and wherein the photovoltaic device does not include a separatesemiconductor absorber layer or a separate semiconductor window layerbetween the buffer layer and CdS_(x)Te_(1-x) layer.
 27. The photovoltaicdevice of claim 26, wherein the CdS_(x)Te_(1-x) layer functions as anabsorber layer.
 28. The photovoltaic device of claim 26, wherein x issubstantially constant over an entire thickness of the CdS_(x)Te_(1-x)layer.
 29. The photovoltaic device of claim 26, wherein x changes over athickness of the CdS_(x)Te_(1-x) layer such that x is larger where theCdS_(x)Te_(1-x) layer is nearer the front contact and x is smaller wherethe CdS_(x)Te_(1-x) layer is nearer the back contact.
 30. Thephotovoltaic device of claim 26, wherein the CdS_(x)Te_(1-x) layer isgraded in sulfur and tellurium content over a thickness of theCdS_(x)Te_(1-x) layer in a step-wise fashion.
 31. The photovoltaicdevice of claim 26, wherein the CdS_(x)T_(1-x) layer is graded in sulfurand tellurium content over a thickness of the CdS_(x)Te_(1-x) layer in agradual fashion.
 32. The photovoltaic device of claim 26, wherein theCdS_(x)Te_(1-x) layer is a copper-doped CdS_(x)Te_(1-x) layer.
 33. Thephotovoltaic device of claim 26, further comprising a semiconductorabsorber layer over the CdS_(x)Te_(1-x) layer, wherein x is greater than0 and less than
 1. 34. The photovoltaic device of claim 33, wherein x isbetween about 0.01 to about 0.30.
 35. The photovoltaic device of claim33, wherein the semiconductor absorber layer comprises sulfur providedby the CdS_(x)Te_(1-x) layer.
 36. The photovoltaic device of claim 33,wherein the CdS_(x)Te_(1-x), layer is a copper-doped CdS_(x)Te_(1-x)layer.
 37. The photovoltaic device of claim 33, further comprising anadditional sulfur-containing layer.
 38. The photovoltaic device of claim37, wherein the additional sulfur-containing layer is positioned betweenthe CdS_(x)Te_(1-x) layer and the semiconductor absorber layer.
 39. Thephotovoltaic device of claim 37, wherein the additionalsulfur-containing layer is positioned within the semiconductor absorberlayer.
 40. The photovoltaic device of claim 37, wherein the additionalsulfur-containing layer is positioned between the semiconductor absorberlayer and a back contact.
 41. The photovoltaic device of claim 37,wherein x is between about 0.05 and 0.5.
 42. The photovoltaic device ofclaim 37, wherein the additional sulfur-containing layer comprises amaterial selected from the group consisting of a CdS_(x2)Te_(1-x2)material (where x2<x), CdS, ZnS and ZnO_(x)S_(y).
 43. A method forming aphotovoltaic device, comprising: depositing a CdS_(x)Te_(1-x) layer overa TCO stack deposited on a substrate, wherein x is greater than 0 andless than 1; and depositing a semiconductor absorber layer over theCdS_(x)Te_(1-x) layer; wherein the CdS_(x)Te_(1-x) layer and thesemiconductor absorber layer are deposited using a VTD coater, andwherein the VTD coater has heater temperature settings such that aconveyance zone prior to a first distribution zone in which theCdS_(x)Te_(1-x) layer is deposited, relative to a direction of movementof the substrate, has a higher temperature at a center of the conveyancezone and a lower temperature at sides of the conveyance zone.
 44. Themethod of claim 43, wherein the VTD coater has heater temperaturesettings such that a pre-heat zone located prior to the conveyance zone,relative to the direction of movement of the substrate, has a highertemperature at a center of the pre-heat zone and a lower temperature atsides of the pre-heat zone.
 45. The method of claim 44, wherein thetemperature at the center of the conveyance zone is about 600° C. andthe temperature at the sides of the conveyance zone is about 510° C.,and wherein the temperature at the center of the pre-heat zone is about625° C. and the temperature at the sides of the pre-heat zone is about535° C.
 46. The method of claim 45, wherein a temperature at sides ofthe first distribution zone is about 640° C. and a temperature at acenter of the first distribution zone is about 610° C.
 47. The method ofclaim 46, wherein the absorber layer is deposited at a seconddistribution zone, located after the first distribution zone, andwherein a temperature at sides of the second distribution zone is about640° C. and a temperature at a center of the second distribution zone isabout 610° C.
 48. The method of claim 47, further comprising depositinga semiconductor window layer over the TCO stack before depositing theCdS_(x)Te_(1-x) layer, wherein the semiconductor window layer isdeposited in a third distribution zone of the coater located betweenfirst zone and the first distribution zone, and wherein a temperature ofthe third distribution zone is about 600° C.
 49. The method of claim 44,wherein the VTD coater further has heater temperature settings such thatthe temperature at the sides of the pre-heat zone is varied, along thedirection of movement of the substrate.
 50. The method of claim 49,wherein the temperature at the sides of the pre-heat zone is varied,along the direction of movement of the substrate, to be about 525° C.,about 575° C., about 650° C., and about 675° C., in order, and thetemperature at the center of the pre-heat zone is about 650° C.
 51. Themethod of claim 49, further comprising depositing a semiconductor windowlayer over the TCO stack before depositing the CdS_(x)Te_(1-x) layer,wherein the semiconductor window layer is deposited in a seconddistribution zone of the coater located between the pre-heat zone andfirst zone of the coater.
 52. The method of claim 51, wherein the VTDcoater further has heater temperature settings such that the temperatureat the center of the conveyance zone is varied, along the direction ofmovement of the substrate.
 53. The method of claim 52, wherein thetemperature at the center of the conveyance zone is varied, along thedirection of movement of the substrate, to be about 560° C., about 590°C., about 650° C., and about 600° C., in order, and the temperatures atthe sides of the conveyance zone are about 550° C.
 54. The method ofclaim 1, wherein the absorber layer is deposited at a third distributionzone, located after the first distribution zone of the coater, andwherein a temperature at sides of the third distribution zone is about630° C. and a at a center of the third distribution zone is about 600°C.
 55. The method of claim 54, further comprising a fourth distributionzone, wherein the absorber layer is deposited at both the third andfourth distribution zones, and wherein a temperature at sides of thefourth distribution zone is about 630° C. and a at a center of thefourth distribution zone is about 600° C.